The vascular endothelium in vivo exists as a non-thromogenic surface to prevent interaction with platelets. However, the extracellular matrix or basement membrane underlying the endothelium is very thrombogenic, and therefore the integrity of the vascular endothelium must be rigorously maintained to prevent unwanted clotting. It is proposed to use a cloned line of adult bovine aortic endothelial cells to elucidate the mechanisms involved in maintaining the integrity of the vascular endothelium and the non-thrombogenicity of the upper endothelial cell surface. Fibronectin, a major cell surface glycoprotein probably involved in cell-cell and cell-substrate adhesion, has recently been identified as a major component of the endothelial extracellular matrix. It is suggested that fibronectin is involved in binding endothelial cells to their extracellular matrix, and that fibronectin may mediate the adhesion of platelets to the extracellular matrix. Therefore, in order to identify the role of fibronectin in endothelial cells, the biosynthesis, secretion, and extracellular location of fibronectin will be studied in cultured endothelial cells, as well as the possible role of fibronectin in platelet adhesion to the extracellular matrix. These studies will involve biochemical, immunological and electron microscopic techniques.